Welcome to the forum! Byron has been a lot of help to me too.
I have a 4 year old as well and he enjoys our fish. He has a Betta that he feeds, with my help. He turns on the light for his tank every day (and leaves the rest alone).
We're going to try our hand at a gecko (or frog) in the new year. We just got a small terrarium. Interested to see how he takes to that.